I have problems with third party books about aircraft system. Especially the US ones, where they explain every button in every detail. I have the book mentioned by Localiser, I flipped it through once and never opened it again.
The main drawback is that they try to teach you the system in another way then Airbus does. Which might be helpful for some, but not for me. Check out your company and their way of asking questions. Mostly, Airbus operators use terms taken directly out of FCOM. Explaining all the buttons on the overhead and giving you nice mnemonics doesn't teach you the Airbus system. The later may be a complicated one, but once comprehensed as an entity not so much.
Nowadays, pilots are no button pusher anymore, they must understand the whole system - or will fail miserably.
Dani